A sapphire substrate, a GaN substrate or the like is used as a substrate for a semiconductor device such as a light emitting diode (hereinafter referred to as LED) or a laser diode (hereinafter referred to as LD).
As the sapphire substrate attains high insulation, it is not possible to provide an electrode on a back surface of the sapphire substrate (referring to a surface of the substrate where a semiconductor layer having a light emission layer is not formed, hereinafter the same as above). Therefore, not only a p-side electrode but also an n-side electrode should be formed on the semiconductor layer which is formed on the sapphire substrate. In such a case, as a result of a current passing through the semiconductor layer having a small thickness, a drive voltage of a light emission device has undesirably been high.
In contrast, since the GaN substrate may be provided with an electrode also on its back surface, the drive voltage of the light emission device can be lowered. Meanwhile, an absorption coefficient of the GaN substrate is larger than that in the sapphire substrate, and a part of light emission is absorbed in the GaN substrate in an LED or the like, which results in lower light emission intensity. In order to solve this problem, a method of manufacturing a GaN crystal substrate attaining high transparency and a low absorption coefficient by using vapor phase growth such as HVPE (Hydride Vapor Phase Epitaxy) as well as a GaN crystal substrate obtained through that manufacturing method have been proposed. The absorption coefficient of that GaN crystal substrate, however, is not sufficiently small (see, for example, Japanese Patent Laying-Open No. 2000-12900 (Patent Document 1)).
Meanwhile, a method of manufacturing a GaN crystal substrate using a flux method in which GaN crystal grows through the steps of accommodating metal Na representing an alkali-metal-element-containing substance and metal Ga representing a group III-metal-element-containing substance in a reactor, melting these metals to form a melt containing an alkali metal element and a group III-metal element, and introducing N2 gas representing a nitrogen-containing substance into the melt has been proposed. The GaN crystal substrate obtained through the flux method, however, is also colored orange or brown, and the absorption coefficient of that GaN crystal substrate is not sufficiently small (see, for example, Hisanori Yamane, et al., “GaN Single Crystal Growth by the Flux Method,” Oyo Buturi, The Japan Society of Applied Physics, May, 2002, Vol. 71, No. 5, pp. 548-552 (Non-Patent Document 1)).
This may be because metal Na representing an alkali-metal-element-containing substance is highly susceptible to oxidation, i.e., an oxygen atom or the like is introduced as an impurity atom, and therefore it is difficult to obtain high-purity group III-nitride semiconductor crystal.
In addition, a nitrogen atom (hereinafter referred to as N atom) is hardly dissolved in a melt of a simple substance of Ga and a melt of a simple substance of Na. Therefore, when metal Ga and metal Na are simply melted, Ga atom and Na atom mix merely slowly with each other and dissolution of the N atom in the melt is not expedited. Namely, it takes a long time before crystal growth of GaN crystal starts.
Therefore, efficient manufacturing of a GaN crystal substrate attaining a low absorption coefficient, which will serve as a substrate for a semiconductor device such as an LED or an LD, has been desired.
Patent Document 1: Japanese Patent Laying-Open No. 2000-12900
Non-Patent Document 1: Hisanori Yamane, et al., “GaN Single Crystal Growth by the Flux Method,” Oyo Buturi, The Japan Society of Applied Physics, May, 2002, Vol. 71, No. 5, pp. 548-552